Bifocal spectacle-glass.



M. VON ROHR.

BIFOGAL SPEGTAGLE GLASS.

APPLICATION FILED JAH.25, 1910.

968,081. Patenfed Aug'.23, 1910. I

UNITED STATES.- PATENT nonrrz vex mun;- ma,

climax, assreitonro 'rin: nan-or cm or .nnu, 'eamnm; 4

mom. sraccracna-eaass.

. A imat mi January, mo. Serial Io. 540,005.

To all whom it may concern: Be it known that I, Monrrz v o'xR'orm, a citizen of the German Em ire, residing at- Carl-Zeiss-Strasse, Jena, in t le Grand Duchy 6 of Saxe-Weimar, Germany, have invented a new and useful Bifocal Spectacle-Glass, of which the followingis a specification.

. The invention consists in a homogeneous bifocal spectacle glass,v both the primary and factory extent corrected for astigmatism.

To attain this object, there has been em.- plo ed' for the primary lens a well known spectacle glass, which has the form of a col lective or dispersive meniscus of such a curvature that it is sufliciently corrected for distant objects as to astigmatism. Astigmatic correction of this-kind can be held to have been attained, at least approximately, when the radius of the convex surface measured in millimeters can be represented as a fraction the numerator of which is1000 and the denominator of which is the sum of two terms, one being the power'of the spectacle glass in dioptriessitive in collective I enses, ne ative in' ispersive lenses-and the other aving a positive value between 10 and 25. It is obvious that such a spectacle glass can be equipped with a secondary lens fornear objects, e. g. for use in readin by replacing a part of one of the two re tiv'e surfaces by a third refractive surface, which in .virtue of, its suitable curvature, forms. together with the opposite part of the unrestricted surface, the sec? ondary lens of the power desired. As always the primary lens is designed for distance vision, the secondary lens for near vision, there belongs to a collective primary lens a still more collective secondary lens and to a di rsive primary lens a. secondary lens WhlGh is less dispersive or, even slightly collective; Hence, whether the primary lens be collective or dispersive, the third surface must deviate, from that part of the one primary lens'su'rface which it is substituted for, toward the air side, covering that part in some mannerwith an additional collective lens. I

The requirement that the secondary lens be likewise sufiiciently corrected for astigmatism is, according to the present invention, fulfilled by bringing the vertex of the collective lens supposed to have been addedthe secondary lens ofwhich are to a. sati's-' so near to the axis of the primary lenslthat the distance between both amounts at the most to 10 min. This vertex is located where the straight line determined by the "lrwll'uflw of hiat- Patented hug-23, 1910. a

center of curvature of the third surface and by the center of curvatureof the. second surface, that surface of the primary lens which surrounds the third surface, pierces the astigmatic correction it is unobjectio'nable to make use, for distance vision, of a field of the spectacle glass above the axis of the primary lens, that is to say, so to place the spectacle glass in front of the eye that the axis of its primary lens, while. assin as usual about through the center 0 rotation of the eye, is directed downward.

As to the astigmatic correction of the secondary lens, it will notbe e ually good, as can .be presumed from the lir'st, with, the two possible arrangements of the third sure face, on the concave or theconvex side of the meniscus. The investigations made in developing the invention have proved this and have at the same time shown, that in this regard the collective and the dispersive menisci have contrary qualities. There results as a uniform rule, that the better astigmatic' correction of the secondary lens is attained, when the second surface, which surrounds the third one, is more curved than {he first surface, which is common to both enses.

third surface. With such an arrangement 5 imary lens can- :in its axis, be-- In the annexed drawing: Figure 1.is a

diagram of a bifocal v spectacle less for myopes constructed according to t e invention. Fig. 2v is a diagram of a bifocal spec-- tacle glass for .hypermetropes constructed according to' the invention.

The dispersive primary lens of the spectacle glass shown 'm Fig. 1 'has a power of 6 dioptr'ie's, whereas the likewise dispersive secondary lens has 3 dioptries. In 2 both are collective,the'; power of the I indicate the two median directions of sight,

which exist in using the primary (upper) of the two surfaces of the primary lens, in

Fig. 1 by C for the radius r of the common outer or first surface and by C. for the radius r. of the inner or second surface, in Fig. 2 by C for the radius 1'? of the common inner or first surface and by C for the radius r, of the outer or second surface. The third surface is produced in Fig. 1 by the radius 1, from the center C in Fig. 2 by the radius r from the center 0 That part of the second surface which has been replaced b the third surface is indicated by a dotte line for the purpose of showin the additional collective lens which woul be bounded by the said art and the third surface. As the axis of t is supposed additional lens is determined in Fig. 1 by the centers C and C and in Fig. 2- by thecenters C and C its vertex has the distance d from the axis of the rimary lens.

In the first example the fo owing measures have been chosen: r =117.7, r,- -49.8, r' =70.1, (lg-17.0) mm. The corresponding measures of the second example are r =7 7 .0, r.=60.0, 1%:44/7, (1:7.0 mm. The radius of the outer surface of the primary lens, '1 in Fig. 1 and r. in Fig. 2, realizes in both cases the condition sti ulated in the first paragraph for a satis actory astigmatic correction of the primary lens. Moreover, the feature that the curvature of the second surface exceeds that of the first surface, being the condition of a more complete correction of astigmatism in the secondary lens, has been embodied in both exam lee.

1. A homo neous meniscal tacle lass which c, mprises a primary ens for that objects and a secondary lens for near objects both corrected for astigmatism, and which .on one side a first surface common to both lenses and on the other side a primarylens surface and a secondary lens surface, this third surface being surrounded by the second surface, the three centers of curvature lying on the same side of the glass and the straight line, determined by the centers of curvature of the second and the third surface, iercing the third surface at a distance. of 10 mm. at most from the axis of the primary lens.

2. A homogeneous meniscal s tacle glass which comprises a (p ant objects and a secon ary lens for near objects, both corrected for astigmatism, and which has on oneside a first surface common to both lenses and on the other side a primarylens surface and a secondary lens surface, this third surface being surrounded by. the second surface, the second surface being more curved tha'n the first one, the three centers of curvature l on the same side of the glass and the straight line, de termined b the centers of curvature of the second an the third surface, piercing the third surface at a distance of 10 mm. at

most from the axis of the primary lens.

, MORITZ VON ROHB.

' Witnesses:

PAUL Knfionu,

Anrnnn Mxcmmz.

rimary ens for v 

